In commonly owned U.S. Pat. Nos. 3,878,485 and 3,973,227, in the name of Paolo Erculiani, there has been described a transmission path of this type comprising several conductors separated by layers of a substantially nonpolarizable dielectric material, such as polyethylene or Teflon, the conductors being much thinner than the invtervening dielectric layers and having a maximum effective thickness on the order of magnitude of the penetration depth .delta. of an alternating current at a frequency in the kHz range. The penetration .delta. is given by a formula disclosed in these two patents.
The conductors referred to can be either cylindrically coaxial or flat (see especially FIG. 7 of each of the two Erculiani patents). Coaxial lines of this description are difficult to manufacture and are therefore generally limited to two cylindrical conductors whereas a flat line can be conveniently assembled from three metal foils with two intervening dielectric layers forming a stack, preferably with conductive interconnection of the two outer foils, as likewise suggested in the Erculiani patents. If it were practical to construct a three-conductor coaxial signal path of this type, the intervening dielectric layers would constitute a homogeneous medium in which the magnetic lines of force from closed loops around the conductor axis. While a stack of flat conductors may be considered part of a coaxial line with infinite radius, the necessarily finite width of the stack creates discontinuities along its sides with resulting nonuniform current distribution within each conductor. In fact, the eddy currents developing along the conductor edges reduce the density of the signal current in these regions whereby the effective line resistance is increased.